Helical steel spring and method

ABSTRACT

This invention relates to a quenched and tempered ferrous alloy suitable for helical springs, such as used in automotive suspension systems. More particularly, it is directed to a hotformed, oil-quenched and tempered helical spring formed of a ferrous alloy consisting essentially of, by weight, about 0.51 to 0.91 percent carbon, about 0.75 to 1.00 percent manganese, about 0.70 to 1.10 percent silicon, about 0.25 to 0.40 percent chromium, the remainder substantially iron and incidental impurities. In the formed and heat-treated condition, said springs are characterized by good resistance to load loss, good resilience, resistance to atmospheric corrosion, and good surface qualilty.

United States Patent 11 1 Furr 1 Nov. 12, 1974 [73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

221 Filed: Nov. 16,1972

211 Appl. No.2 307,282

Samuel T. Furr, Emmaus, Pa.

[52] us. Cl 148/2, 75/126 R, 148/12, 148/36 51 Int. Cl. c2111 9/02, C21d 7/14, C22C 39/44 [58] Field of Search 75/126 R, 126 A, 126 Q; 148/36, 2, 12

[56] References Cited UNITED STATES PATENTS 1,972,524 9/1934 Kinzel 148/36 2,028,029 1/1936 Van Royen 75/126 R OTHER PUBLlCATIONS Metal Selector, 1963 Edition, page 8-25. Metals Handbook, Vol. 1, 8th Ed., l96l pgs. 160-165. Journal of the Iron and Steel lnstitute, April 1969,

pgs. 461-467. Alloy Digest Filing Code SA-70, AlS1-9261, June 1958.

Primary Examiner-C. Lovell Attorney, Agent, or Firm-Joseph .l. OKeefe; William B. Noll [5 7 ABSTRACT This invention relates to a quenched and tempered ferrous alloy suitable for helical springs, such as used in automotive suspension systems. More particularly, it is directed to a hot-formed, oil-quenched and tempered helical spring formed of a ferrous alloy consisting essentially of, by weight, about 0.51 to 0.91 percent carbon, about 0.75 to I00 percent manganese, about 0.70 to H0 percent silicon, about 0.25 to 0.40 percent chromium, the remainder substantially iron and incidental impurities. In the formed and heattreated condition, said springs are characterized by good resistance to load loss, good resilience, resistance to atmospheric corrosion, and good surface qualilty.

2 Claims, 3 Drawing Figures P HUY12|974 I ATENTED sum NF 2 3.847.678

V INV. HIGH CARBON 0 INV. LOW CARBON 0 5 O 5 0 w w 5 0 PERMANENT SET DEGREES PATENTEUHnv 12 mm STRENGTH p.s. i.

sum EM 2 3'847-678 O INV. 0.60 CARBON V INV. 0.80 CARBON L AISI 9260 E] AISI 5|60 42 44 4e 48 so 52 54 56 HARDNESS RC 1 HELICAL STEEL SPRING AND METHOD BACKGROUND OF THE INVENTION This invention in general is directed to hot-formed helical steel springs. Specifically, the invention is di-. rected to helical steel springs which are hot-formed from as-rolled bar stock into helical shape and oilquenched and tempered, shot-peened and cold set to obtain the desired properties.

Traditionally, hot-formed helical steel springs, such as used in automotive suspension systems, have been made from AISI alloy steel grades 5160 and 9260. AISI alloy steel grade 5160 is hot rolled into bar stock having a surface quality which does not require surface treatment prior to use. The as-rolled bar stock is hotformed into helical steel springs which are oilquenched and tempered. The helical steel springs so produced are susceptible to sagging due to load loss. Load loss can be defined as a weakened ability of the helical steel spring to recover from high stressesand strains. The steel in the helical spring has relatively poor resiliency, hence suffers load loss. On the other hand, AISI alloy steel grade 9260 is difficult to roll into bar stock having good surface quality. Further, said steel has a reputation for decarburization problems. As a result, the as-rolled bar stock must be subjected to a surface treatment, such as centerless grinding, prior to hot-forming into helical springs. The surface treatment removes excessive decarburization, pits and scams which make the helical springs sensitive to cracking during oil-quenching and tempering, which individually or in combination may adversely affect the fatigue life, load-loss capabilities and resiliency of the helical springs. 8

Prior art practices as exemplified in U.S. Pat. No. 1,972,524 issued Sept. 4, 1934 to Augustus BsKinzel entitled Alloy Steel Spring are directed to the modification of medium carbon silico-manganese steels containing large amounts of manganese for use as springs. Molybdenum is substituted for chromium and small amounts of vanadium are added to produce airhardenable steels which are less susceptible to cracking during quenching than are oil-hardening steels. These steels are used to manufacture springs, such as leaf springs, but are not generally hot formed into helicaltype springs. Another prior art practice, as exemplified in US. Pat. No. 2,155,348 issued Apr. 18, 1939 to Marcus A. Grossman entitled Alloy Steels is directed to the addition of an element, such as titanium, to a carbon-manganese-silicon-chromium steel to make a shallow-hardening fine-grain steel. While the steels so modified are satisfactory for springs which can be coldformed, they are not satisfactory for highly stressed larger diameter hot-formed helical springs.

In contrast to the latter steels, the ferrous alloy of this invention may be hot-formed to give excellent inservice performance.

SUMMARY OF THE INVENTION I The chemistry is balanced such that it falls essentially within the following range, by weight:

Carbon 0.51% to 0.9l% Manganese 0.75% to 1.00% Phosphorus 0.035% max. Sulfur 0.040% max. Silicon 0.70% to 1.10% Chromium 0.25% to 0.40% Iron balance.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a perspective view of a helical spring made according to the teachings of this invention.

FIG. 2 is a graph comparing the resilience of the alloy of this invention with two conventional spring steels.

FIG. 3 is a graph comparing the tensile strength of the alloy of this invention with the tensile strengths of the same steels graphically illustrated in FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT This invention relates to an oil-quenched and tempered ferrous alloy particularly suitable as a helical spring for application in an automotive suspension system. Such springs, a typical one being illustrated in FIG. 1, can be hot-formed from as-rolled bar stock. The helical steel springs are then oil-quenched and tempered to a desired hardness of about R 48 to R 52, shot peened and preset. In said condition, the springs are characterized by an improved resiliency and resistance to load loss. Said springs are made of a ferrous alloy having a chemical composition within the following range, by weight:

Carbon 0.51% to 0.91% Manganese 0.75% to 1.00% Phosphorus 0.035% maximum Sulphur 0.04% maximum Silicon 0.70% to 1.10% Chromium 0.25% to 0.40%;

the remainder essentially iron and incidental impurities. For reasons to become apparent hereinafter, it is preferred to limit the carbon to a maximum of 0.76 percent.

It has been found that either carbon or silicon by itself, or combinations thereof, can affect the resiliency and therefore the load-loss capabilities of the helical steel springs. Whereas an increase in either carbon or silicon content increases both the resiliency of the helical springs and resistance of the helical springs to loadloss, the presence of both carbon and silicon has a synergistic effect on the resiliency and load loss capabilities of the said helical springs. Relatively high carbon content results in higher quenched hardness of the steel and increases the susceptibility of the steel to crack during quenching from the austenitizing temperature. The carbon is, therefore, carefully controlled to obtain the highest hardness compatible with increased resiliency and little if any increase to the susceptibility of cracking during quenching. The silicon content of the steel must also be carefully controlled. While silicon increases the resiliency of the steel, aids in increasing the shock resistance of the alloy, and increases the alloys resistance to atmospheric corrosion, excessive silicon can promote graphitization and adversely affect resistance to decarburization during hot forming and in resiliency and hence resistance to load-loss of the helical springs is increased without detrimentally affecting the fatigue life, shock resistance, and other mechanical properties thereof. The steel can be hot-formed into helical springs from as-rolled bar stock because the surface quality of the bar stock is not seriously affected by the carbon and silicon contents. The hot-formed helical springs have good resistance to cracking when oil-- quenched from the austenitizing temperature, for example, l,500 F. to l,700 F. As noted previously, the addition of carbon and silicon have a synergistic effect on the resiliency and load-loss capabilities of the steel, therefore, it is preferred to hold the carbon to a maximum of about 0.76 percent, by weight.

While the manganese and chromium contents have little if any effect on the resiliency of the helical springs, the elements do add to the hardenability of the steel and within the ranges specified have little or no affect on the retention of austenite after tempering.

Additionally, manganese aids in promoting ease of rolling and maintains a desired manganese to sulfur range in the steel. The chromium increases the hardenability of the alloy and its resistance to atmospheric corrosion.

The alloy of this invention has sufficient hardenability to be heat treated, for example, by oil-quenching and tempering in the hot-formed condition to a hardness within a range of about R 48 to 52. Larger amounts of manganese and chromium than specified above would increase the hardenability of the steels. Further, larger amounts of said elements are not desirable because of the increase in the cost of the product and because of the increase in the susceptibility of the steels to quench cracking.

Helical springs used in automotive suspension systems are normally tested for resiliency, load-loss, mechanical properties, fatigue life, impact and hardenability. All the tests are generally standard tests of short duration. However, the resiliency tests and load-loss tests are traditionally made on hot-formed, heat treated helical springs of actual size which are placed under a known load for a relatively long specified period of time, i.e., 5 weeks or longer. It will be understood that for such conventional testing, the actual production of helical springs and considerable time is necessary to fully evaluate the capabilities of the material for use as helical springs. Now, as with the determination of other long term performance properties of amaterial, such as corrosion or atmospheric exposure resistance, simulated or accelerated tests have been devised to accurately measure the long term performance of a steel for use as a helical spring. I

For helical springs, such as described herein, the torsional Bauschinger test was developed toevaluate the resiliency and the potential load-loss resistance of spring steels. A large Bauschinger effect means increased resilience and consequently improved resistance to load-loss. However, before describing the test procedure, it may be helpful to review those properties of a steel which make it suitable for helical springs.

To begin this description, we may first isolate the particular property that accounts for load-loss resistance. Since springs are designed to store large amounts of elastic energy, spring steels must have a large resilience, i.e., a large energy-storage capacity. Resilience is the ability to absorb internal work or energy or, in corollary terms, the ability of a material to recover its original size and shape after deformation caused by stress. Assuming quenched-and-tempered materials of equal strength and hardenability, this property is measured by the modulus of hyperelastic resilience, which is the greatest amount of energy that can be stored in a material without causing plastic strain, i.e., permanent set. The important feature of the modulus of hyperelastic resilience is that it measures energystorage capacity after a preset. Since the final steps in the spring making procedure are typically shot peening and cold setting, the most important concern lies in measuring the resiliency after plastic strain. As previously-noted, one symptom of load loss is a decrease in free length of the spring. Since a decrease in free length means that the spring has suffered plastic strain, it can be concluded that load loss occurs when there is an attempt to store more energy in the spring than the resilience of the steel will allow. In any case, the torsional Bauschinger test can be used to rate the potential loadloss resistance of spring steels, since the steel with a larger Bauschinger Effect has a larger modulus of hyperelastic resilience and consequently improved resistance to load loss.

The Bauschinger Effect, i.e., the weaking of the metal when it is strained in a reverse direction after a plastic strain in the forward direction, is measured on a standard torsion testing machine. The torsional Bauschinger specimen has a gage length and diameter of 3 inches and 0.357 inches, respectively. These torsion test specimens are prepared as follows: rough machine 0.050 inch oversize, oil quench and temper to the usual hardness of springs 50 Rockwell C, and grind to size. The ground surface of this specimen is similar to the surface of springs manufactured from centerlessground bars. With such specimens there is also better control of process variables such as hardness and amount of preset. In turn, this improved control makes it possible to determine the relationship of these variables to the load-loss problem. For a more detailed review of this method for predicting relative load-loss resistance of steels, attention is directed to the article, by the inventor herein, in the March 1972 Transactions of the ASME in the Journal of Basic Engineering.

Turning now to the graphic illustrations of FIGS. 2 and 3, it will be seen that FIG. 2 is a graph showing the actual Bauschinger twist in degrees and the permanent set in degrees in the test specimens of AISI alloy steel grades 5160 and 9260 steels and high carbon and low carbon steels of the invention. Curve A shows the re sults of testing a high carbon steel of the invention and AISI alloy grade 9260. Curve B shows the results of testing a low carbon steel of the invention and curve C shows the results of testing AISI alloy grade 5160. Obviously the high carbon steels of the invention and AISI alloy grade 9260 are equivalent for this particular showing. Both of the steels show results better than the low carbon steel of the invention and AISI alloy grade 5160. In turn, the low carbon steel of the invention shows results better than AISI alloy grade 5 160. It must be remembered that while the test results show AISI alloy grade 9260 to be equivalent to the high carbon steel of the invention and better than the low carbon steel of the invention, the steels of the invention can be used in the hot rolled condition whereas bar stock and hot rolled at a temperature of about 2,050 F. into bars 0.53 of an inch in diameter. The bars were sheared into lengths of about 103 inches. The bars were divided into test specimens. The fracture grain size of the steel The steel was hot rolled into 4 1: inches by 4 A: inches billets. The billets were conditioned by surface grinding rolled from AISI alloy grade 9260 steel must be surface 5 was determined by oil quenching specimens 3 inches in ground prior t h t-fo i g a d h t t ti g into u length from several austenitizing temperatures as folable helical springs. IOWSI A comparison of the tensile properties of the steels 0f the invention and A181 alloy grades 5160 and 9260 Austenitizing Fracture Grain Size I is shown in FIG. 3. Note that the mechanical properties (shephe'd Stan Rt are essentially equal at the operating hardness of RA8 1400 8 5m to R50. 1450 s 64.9 As noted above, the steels of the invention are sub- Egg 3 22-3 stantially free of decarburization and have good surface 1600 a 64:4 quality. While no surface treatment, such as grinding, 123g gig is necessary, the helical springs are shot-peened and 1750 preset to aid fatigue life. The steels can be hot-formed in the as-rolled condition. A comparison of the decarburization of A181 alloy grade steels 5160 and-9260 and A tempering series was conducted on specimens 3 steels of the invention are shown in Table 1. Specimens inches long and oil quenched from l,600 F. Each specof each grade were turned down to the same size, that imen was heated for one hour at a temperature listed is, 0.650 of an inch in diameter, to remove all mill debelOWl carburization. The specimens were placed in a furnace and were heated to l,950 F. for one hour in an atmo- T T H d R sphere of air bubbled through water to simulate moist empenng empemure at air. The specimens were oil quenched, tempered to a 600 56.7 hardness of R 50 and tested for decarburization by mi- 2, ,8 22'? croscopically examining each at a magnification of 100 750 5210 diameters. The results are shown below in Table 1. 32g 22-: TABLE! :23 21;:

Average Decarburization of Steels Used to Manufacturing Springs 35 Free Ferrite Maximum Affected The hardenability test on reduced size, 5% of an inch Steel Grade Depth (inch) Depth (inches) diameter specimen showed the hardness drop from R 635, 1/16 of an inch from the quenched end, to R, 3:28 Z8823 2:3; 48.7 at 13/16 of an inch from the quenched end. The Steel of invention .0010 .0116 40 steel had sufficient hardenability to be used as a helical spring. The tensile properties of the steel were determined Obviously AISI steel" alloy grade 9260 is more suscepon 0.357 of an inch diameter specimens with 1.4 inch tible to decarburization than either of the other two gage length. The specimens were rough machined steels and that AISI steel alloy grade 5160 and the 0.050 inch oversize, austenitized at 1,600 F. for k steels of the invention are compatible. hour, oil quenched and tempered to obtain a hardness In a specific e ample of the invention, a heat of steel of about R 50, and ground to size. The results follow:

Yield Tensile Impact Tampering Strength Strength Elong. R. in A. (V-notch) Temp. (F.) Hardness R (0.2% Offset) (psi.) (Charpy-Ft-Lbs) was melted and had the following chemical composi- Specimens oil quenched from l,600 F. and temtion, by weight: pered at 775 F. and 825 F. were tested by a torsional Bauschinger test to predict the potential load-loss resis- Carbon 0.61% Mangmme 091% tance. A large Bauschinger Effect means increased re- Phosphorus 0.009% silience and improved resistance to load loss. The ratio 88%;? of an actual Bauschinger Twist to Permanent Set mea- Chromium sured after a twist of 55 in the forward direction is a Iron balance measure that can be used to compare the resilience of the steel. A desirable ratio is one falling within the range of about 0.85 to 1.10. The results of the tests are shown below:

Tempering Temperature Hardenability (F.) Hardness R, Ratio l/l6ofanlnch 13/16 ofan lnch 775 50.2 0.95 from Quench End from Quench End M Yield Tensile Tempering Strength Strength Elong. R. of A. Impact (V-notch) Temp. (F.) Hardness (0.2% Offset) (psi.) (70) (Charpy-Ft-Lbs) Resiliency Test Tempering Temperature (F.) Hardness Ratio In another specific example of the invention, a high carbon heat of steel was melted having the following .chemical composition, by weight:

Carbon 0.80% Manganese 0.91% Phosphorus 0.010% Sulfur 0.015% Silicon 0.90% Chromium 0.36% Iron Balance The steel was hot rolled and tested as noted in the first specific example. The results of the tests are listed be- 1. A quenched and tempered helical spring, hotformed from as-rolled bar stock, characterized by a hardness between about R 48 and R 52 and a high resistance to load loss as exemplified by a Bauschinger Twist/Permanent Set Ratio between about 0.85 and 1.10, and consisting essentially of, by weight, about 0.76 to 0.91 percent carbon, about .75 to 1.00 percent manganese, about 0.70 to 1.10 percent silicon, about low: 0.25 to 0.40 percent chromium, with the balance iron Fracture and incidental impurities. Oil Quench l-rom Fracture L iftflf i'gg' j Hardness RC 40 2. In a method for producing a helical steel spring for use m automotive suspension systems wherein a steel is 1400 5% 222 melted, cast, solidified, hot-rolled into bar stock, hot- :238 3 formed in as-rolled condition into said helical steel 1550 8- /6 662 spring and heat treated to a hardness range of R 48 to 1228 3 22:3 R SZ, the improvement comprising in combination 1700 7 65.0 therewith the steps of hot-forming a steel consisting es- 1750 6 sentiall of, b wei ht, about 0.76 to 0.91 ercent car- Y y g P Tempe, series bon, about 0.75 to 1.00 percent manganese, about 0.70 to 1.10 percent silicon, about 0.25 to 0.40 percent p s g rp Hardness Re 50 chromium, remainder iron and incidental impurities, wherein said helical steel spring is characterized by a 600 high resistance to load loss as exemplified by a Bausch- $38 1 inger Twist/Permanent Set Ratio between about 0.85 750 53.6 and 1.10. 800 55 s 850 490 a a a c 900 46.7 950 V 512. 

1. A QUENCHED AND TEMPERED HELICAL SPRING, HOT-FROMED FROM AS-ROLLED BAR STOCK, CHARACTERIZED BY A HARDNESS BETWEEN ABOUT RC48 AND RE52 AND A HIGH RESISTANCE TO LOAD LOSS AS EXEMPLIFIED BY A BAUSCHINGER TWIST/PERMANENT SET RATIO BETWEEN ABOUT 0.85 AND 1.10, AND CONSISTING ESSENTIALLY OF, BY WEIGHT, ABOUT 0.76 TO 0.91 PERCENT CARBON, ABOUT .75 TO 1.00 PERCENT MANGANESE, ABOUT 0.70 TO 1.10 PERCENT SILICON, ABOUT 0.25 TO 0.40 PERCENT CHROMIUM, WITH THE BALANCE IRON AND INCIDENTAL IMPURITIES.
 2. In a method for producing a helical steel spring for use in automotive suspension systems wherein a steel is melted, cast, solidified, hot-rolled into bar stock, hot-formed in as-rolled condition into said helical steel spring and heat treated to a hardness range of Rc48 to Rc52, the improvement comprising in combination therewith the steps of hot-forming a steel consisting essentially of, by weight, about 0.76 to 0.91 percent carbon, about 0.75 to 1.00 percent manganese, about 0.70 to 1.10 percent silicon, about 0.25 to 0.40 percent chromium, remainder iron and incidental impurities, wherein said helical steel spring is characterized by a high resistance to load loss as exemplified by a BaUschinger Twist/Permanent Set Ratio between about 0.85 and 1.10. 